Date of Award
Doctor of Philosophy (PhD)
Mark E. Roberts
Amod A. Ogale
Mark C. Thies
Over the past decade, the production of biofuels from lignocellulosic biomass has steadily increased to offset the use of fuels from petroleum. To make biofuels cost-competitive, it is necessary to add value to the “ligno-” components (up to 30% by mass) of the biomass. Instead of just burning it for heat and electricity generation, direct conversion of lignin carbon into a product provides a complementary path to carbon capture, promoting a more environmentally conscious approach. However, the utilization of these lignin-rich byproduct streams, which are separated from the biofuels production, for industrial products can be challenging. The use of specific separation processes results in the lignin-rich byproduct streams containing varying amounts of sugars (cellulose and hemicellulose) and ash. Moreover, the molecular properties of lignin are complex, with significant variations such as chain size and functionality. Although researchers have developed methods to extract lignin from the lignin-rich byproduct streams and try to incorporate it into some industrial products, this approach does not fully utilize the carbons of the lignin-rich byproduct streams, particularly the sugars.
Activated carbon (AC) is a promising product that can be made from the lignin-rich byproduct streams, due to its large and continually growing market. Previous studies on producing AC from lignin-rich byproduct streams have primarily focused on varying activation/carbonization conditions of sugars-separated lignin streams. This approach results in AC with low yields (below the goal of US Department of Energy of 50% carbon utilization) or poor AC properties, like low surface area. This is because these studies have not thoroughly investigated the relationship between the composition of lignin-rich byproduct streams, properties of materials in the streams, and the properties of the resulting AC. Furthermore, a considerable amount of lignin is lost as it is separated from sugars in the lignin-rich byproduct streams, further decreasing the amount of product obtained.
In pursuit of the goal of fully utilizing the lignin-rich byproduct streams of the biorefineries, this dissertation provides a comprehensive analysis of the relationship between the properties of lignin-rich byproduct streams, including the sugars and ash content, molecular weight (MW) of lignin, and the properties of the resulting AC. This is made possible using a novel Aqueous Lignin Purification with Hot Agents (ALPHA) process that was developed to separate the lignin-rich byproduct streams and obtain lignin fractions with varying sugars and ash contents, and clean lignin fractions of different MWs. AC was synthesized using ZnCl2 activation process with low temperature carbonization, which allowed us to obtain carbon conversions of more than 65%.
For AC synthesized from lignin-rich byproduct streams derived from hybrid poplar (HP), the sugar content is the primary factor defining the surface area and pore structure of the resulting AC. As the sugar content increases from 0 (pure lignin) to 55.6%, the surface area increases linearly from 1400 to 2500 m2/g. AC with both a high surface area of 2500 m2/g and a high carbon conversion of 76.0% is achieved in the lignin sample with the sugar content of 55.6%. For nearly pure lignin samples, a higher ash content was observed to promote pore enlargement in AC. Considering that sugars and ash also are components of the lignocellulosic biomass, the results we obtained about the effect of sugars and ash content in lignin sample on AC properties clarify the effect of biomass composition on AC properties, which have not shown consistent results in literature until now. Clean lignin fractions were created from the HP with MW range from 0.8 x 104 Da to 2.2 x 104 Da. This led to total pore volume of the AC with a range from 0.76 to 0.87 cm3/g. Additionally, we observed that the surface area of the AC increases with the increase of the water ratio in the ALPHA solvent. However, the average pore width decreases as the water ratio decreases.
As a comparison, Kraft lignin was purified and separated, producing fractions with MW in a range of 1.2 x 104 to 4.4 x 104 Da. Both the surface area and total pore volume of AC increase with the increase in MW of lignin. The surface area varied from 1420 to 1830 m2/g corresponding to the increase in MW of lignin. Moreover, significant pore enlargement was noted in the lignin sample with low MW, which resulted in a larger average pore width. The rationale for this difference requires further study.
Our new knowledge of how the sugars and ash, and how MW of lignin affect AC properties is extremely important and it fills the knowledge gap between the properties of the lignin-rich byproduct streams and the properties of AC. This knowledge can be used to guide the industrial production of the AC from the lignin-rich byproduct streams. The properties of the resulting AC can be predicted based on the properties of the fractions of the lignin-ruch byproduct streams, assuming a given constant activation/carbonization process. Furthermore, these relationships can also assist in the selection of AC precursors for target applications.
Wu, Chengjun, "Activated Carbon Synthesis From Lignin-Rich Byproduct Streams of Biorefineries" (2023). All Dissertations. 3440.
Available for download on Saturday, August 31, 2024